Silicon's volume expansion during lithiation make its use as a battery material problematical. While the commercialized graphite electrode expands roughly 10-13% during lithium intercalation, silicon's expansion amounts to nearly 300%, generating structural degradation and instability of the solid-electrolyte interphase. Such instabilities ultimately shorten the battery life to inadequate levels, cause breaking of conduction channels, active material isolation, continuous solid-electrolyte interphase reformation, and a mechanically unstable solid-electrolyte interface.
The solid-electrolyte interphase layer forms on the anode surface through reductive decomposition of the electrolyte during charging of the battery. Silicon anodes suffer from a dynamic solid-electrolyte interphase that must reform each cycle as expansion during lithiation causes the layer to break. Formation of the solid-electrolyte interphase consumes Li+ and depletes electrolyte during every cycle. In contrast to half-cells, which utilize a Li metal counterelectrode with an effectively unlimited supply of Li+, full-cells have a limited supply of Li+ provided by the cathode. It follows that the continuous breaking and reforming of the solid-electrolyte interphase layer quickly destroys the cell's cycling performance.
Alternative electrolyte compositions and active material surface treatments have been studied in the effort to enhance SEI formation on high-capacity anode materials and improve half-cell CEs. In spite of these efforts, the CEs achieved throughout cycling are still insufficient for a long-lasting Si-based full-cell or the methods employed to manufacture the full-cells introduce large excesses of Li+ (>200%) into the system that camouflage its true performance. In the effort to design next generation electrolyte materials, room temperature ionic liquids (RTILs or ILs) are of interest due to their low volatilities, negligible vapor pressures, thermal stabilities, high voltage stability windows, and sufficient ionic conductivities. RTILs, particularly those consisting of the pyrrolidinium (PYR1n+) or 1-ethyl-3-methyl-imidazolium (EMIM+) cation and the bis(trifluoromethanesulfonyl)imide (TFSI−) or bis(fluorosulfonyl)imide (FSI−) anion, are cathodically stable with popular negative electrode materials including Si. A clear understanding of the electrochemical properties and interfacial chemistries of these materials has not yet been developed. Little work has been dedicated to the study of the compatibility between RTIL electrolytes and Si-based nanocomposite electrodes, with published work in this field, to date, investigating Si-RTIL systems in thin film type electrodes.
For these and other reasons there is a need for battery materials capable of providing high energy-density and long cycling life at low cost.